[Note: The free full text PDF of this article is available here. It reports on a first-time analysis of several large populations living at latitudes in the Northern Hemisphere where UV light is insufficient to produce vitamin D in an expectant mother's skin between October and March. Significantly greater risk of MS was found in people born at those latitudes during April and May (after the winter months) than for those born during October and November (after the summer months).]

Background: Month of birth has previously been described as a risk factor for multiple sclerosis (MS). This has been hypothesized to be related to maternal vitamin D levels during pregnancy, although conclusive evidence to support this is lacking.

To date, no large studies of latitudinal variation in the month of birth effect have been performed to advance this hypothesis.

Methods: Previously published data on month of birth from 151,978 MS patients were compared to expected birth rates. A linear regression model was used to assess the relationship between latitude and observed: expected birth ratio of MS patients for each month.

Results: Analysis of all reported data demonstrated a significant excess of MS risk in those born in April (observed-to-expected 1.05, p=0.05) and reduction in risk in those born in October (0.95, p=0.04) and November (0.92 p=0.01).

A conservative analysis of 78,488 patients revealed an excess MS risk in those born in April (1.07, p=0.002) and May (1.11, p=0.0006), and a reduced risk in those born in October (ratio 0.94, p=0.004) and November (0.88, p=0.0002). [A risk ratio of 1.0 would indicate no difference in risk; thus the risk ratio of 1.11 for May would indicate risk above expected of 11%, and the 0.88 ratio for November would indicate 12% less risk than expected for November births.]

A significant relationship between latitude and observed-to-expected ratio was demonstrated in December, and borderline significant relationships in May and August.

Conclusions: Month of birth has a significant effect on subsequent MS risk. This is likely to be due to ultraviolet light exposure and maternal vitamin D levels, as demonstrated by the relationship between risk and latitude.